The present invention relates generally to extending transducer head lifetime, an in particular to setting GMR head bias current in disk drives to ensure adequate GMR head lifetime.
Many disk drives utilize giant magneto resistive (GMR) transducer heads for recording data to, and reading data from, magnetic media on data disks. The GMR head performance and lifetime is a function of the temperature of the head. The readback signal amplitude degradation in GMR heads is a function of age or stress in the heads. As recording density increases steadily, readback signal amplitude degradation in GMR heads becomes a very serious drawback.
Further, to meet signal to noise ratio requirements for higher density recording and readback, it is desirable to provide either higher GMR sensitivity (to offset the GMR signal degradation due to reduction of track width shrinkage in higher density recording), or a shorter stripe in the heads to maintain head sensor aspect ratio. Because developing higher sensitivity GMR requires tremendous effort and time to develop, for practical reasons, conventionally the GMR sensor stripe height is shortened to keep up with recording density increases.
The bias current density in the stripe film increases accordingly with the stripe height reduction. Historically as recording density increases steadily, the current density in the sensor has increased from low 106 A/cm2 in early GMR programs to about 4xc3x97108 A/cm2, (in the Cu spacer layer) in the recent GMR designs. However, when the current density in the sensor stripe increases, anti-ferromagnet (AFM) de-pinning and interlayer diffusion become more severe due to the accompanied stripe temperature increase. In addition to temperature increase, electromigration and AFM de-magnetization caused by increased current density contribute significantly to amplitude degradation. For example, at stripe temperature of 168 C., a 14% amplitude loss is observed in GMR heads stressed by a 4.5 mA bias current, and a 41% amplitude loss is observed in GMR heads stressed by a 5.0 mA bias current.
Conventionally, the head bias current is adjusted based on temperature for optimizing head performance and GMR lifetime in disk drives. The value of the bias current used in disk drives is determined by the GMR sensor stripe temperature, without a systemic treatment of the effects of current density on GMR lifetime.
There is, therefore, a need for a system and method for providing correct bias current setting scheme for transducer heads in disk drives based on the resistance of each individual head measured by establishing bias current dependence of GMR resistance and GMR lifetime.
In one embodiment, the present invention provides methods to establish dependence of head lifetime on bias current (i.e., time to failure-TTF) and head resistance specification, to meet head lifetime requirements, by taking both head (i.e., stripe) temperature and bias current into consideration.
The head bias current is adjusted based on individual head resistance and temperature for optimizing head performance and GMR lifetime. Generally, lower bias current is applied to GMR sensors with high resistance to avoid high sensor temperature and to prolong GMR lifetime.
A method is provided for determining correct bias current setting (level) in a disk drive based on the resistance of each individual head, by establishing: (1) dependence of GMR resistance on bias current, and (2) dependence of lifetime on bias current. The bias current setting processes can be programmed into each disk drive and can be different in the details for each GMR head or disk drive supplier.
Further, a method is provided for GMR lifetime test at the drive level (i.e., after drive assembly) based on the established bias current dependent lifetime.